Endosymbiosis Case Study

Today, we will learn about Endosymbiosis. Evolution is one of the major pillars of the AP Biology exam. According to the CollegeBoard course curriculum, evolution is the first “big idea” that you must know for the AP Bio exam. Big idea one tells us that, “the process of evolution explains the diversity and unity of life, but an explanation about the origin of life is less clear”. It goes on to say “mutually beneficial associations among ancient bacteria are thought to have given rise to eukaryotic cells.” In this AP Biology Crash Course we will study this part of the first big idea.

It is very important for AP Bio students to understand how organisms change over time. In order for you to understand evolution, you must study the beginnings of eukaryotic life. The beginning of eukaryotic evolution began with the first eukaryotic cell. Endosymbiosis, a theory accepted by most biologists, states that eukaryotic cells emerged from the engulfing of prokaryotic cells. There is a lot of support that scientists have which allows them to accept this theory. In this article, Endosymbiosis: AP Biology Crash Course, we will start by reviewing the theory itself. We will then present the evidence that scientists have collected in support of the theory. Finally, we will go over a free response question that was seen on the 2011 AP Bio exam.

What is Endosymbiosis?

Endosymbiosis is an evolutionary theory which posits that eukaryotic cells arose from prokaryotic cells. The theory further articulates that the mitochondria found in today’s eukaryotes evolved from aerobic bacteria and that the chloroplasts found in today’s plant cells arose from cyanobacteria.

When life started some estimated 3.6 billion years ago, there were only prokaryotes. Scientists do not believe eukaryotes evolved until about 2 billion years ago. The theory is that aerobic bacteria and cyanobacteria were engulfed by larger cells. The larger host cells benefited from the presence of the bacteria and the bacteria benefited from living inside of the host cell.

It may seem strange that the bacteria that was engulfed benefited, but the world 2 billion years ago was dangerous for a small bacteria. By being engulfed by the larger bacteria, the smaller bacteria were able to survive and flourish. The larger bacteria gained the ability to manufacture energy from oxygen (mitochondria) or sunlight (chloroplasts). This symbiotic relationship led to the incorporation of the mitochondria and chloroplasts in the cell.

History of Endosymbiosis Theory

The theory was first articulated by the Russian botanist Konstantin Mereschowski. He was familiar with the work of Andreas Schimper who had observed the division of chloroplasts in green plants and thought they resembled free-living cyanobacteria. The idea was further advanced and researched later by Dr. Lynn Margulis (this is a name you may want to know for the AP Bio test). She had a hard time getting this published, though and was criticized for a long time as the scientific community had some trouble believing that single-celled organisms were engulfed by other single-celled organisms, and eventually this is what led to the very complex organisms that we have today. After more work and discussion, though, we arrive today to find it generally accepted.

Evidence of the Endosymbiosis Theory

One of the learning objectives of AP Bio is to be able to evaluate evidence provided by data from many scientific disciplines that support biological evolution. In this section we will review evidence of the endosymbiosis theory.

There is a fair amount of evidence to point to the organisms that once lived freely and were engulfed becoming organelles as we know them today. There is a type of bacteria, Rickettsia prowazekii, which looks very similar to mitochondria we currently have. Both have double-membranes and look similar under a microscope. Though looking similar was not enough evidence for scientists to conclude that endosymbiosis had occurred. In order to support the theory, scientists had to delve much deeper into the organism.

First, new mitochondria and chloroplasts are only able to arise from pre-existing mitochondria and chloroplasts. When a cell divides, the mitochondria and chloroplasts must divide themselves. The DNA that encodes for the creation of the proteins necessary to build the mitochondria and chloroplasts is only found within their unique genome.

The genome that mitochondria and chloroplasts have compares more closely to the genome of a bacterium than the nuclear genome. The nuclear genome, as you may recall, is made up of double stranded, linear DNA. The genome of a bacterium, however, is made up of single stranded, circular DNA. Additionally, the genome of bacteria has no histone proteins that are found in nuclear DNA. The genome of mitochondria and chloroplasts is also single stranded and circular with no histones.

When scientists looked more closely at the genome of the mitochondria and chloroplasts, they also found that the amino acid sequences more closely resemble a bacterium’s sequence. The first amino acid in bacteria is methionine with a formyl group added to it. This amino acid is also the first transcript in the mitochondria and chloroplast.

Additionally, when scientists treated the mitochondria and chloroplasts with antibiotics, the antibiotics were able to control their replication in the same mechanism used to control bacteria replication. One specific antibiotic called rifampicin is given to patients to prevent the RNA polymerase of bacteria from properly functioning. By inhibiting the function of RNA polymerase, the bacteria will not be able to replicate or repair DNA causing the bacteria to die. When scientists treat mitochondria with rifampicin, similarly, the mitochondria is not able to reproduce and dies. When we treat the nucleus with the very same antibiotic, there is no adverse reaction, and the DNA is able to be replicated.

Just in case you are not convinced, here is one final piece of evidence. When the mitochondria are reproducing, they perform binary fission. Bacteria reproduce by “pinching” in half in a process that is named binary fission. Similarly, when mitochondria reproduce they separate using the very same mechanism.

Endosymbiosis in Coral

One place we can easily observe this process occurring in nature today is with certain types of coral. Coral is an animal thing that can use the process of photosynthesis to make energy. How is this possible? Symbiodinium is a dynoflagellate that is eaten by the coral. As an algal organism, it has the chloroplasts to carry out photosynthesis. The coral allows it live within, and in return, it allows the coral to use its photosynthetic process to obtain energy. So the coral feeds itself continually through photosynthesis carried out by the other organism.

AP Biology Exam Question

Now that we have reviewed what the endosymbiosis theory is and how it is supported let’s study how you might be asked about it on the AP Bio exam. First, we will go over this free response question:

According to the endosymbiotic theory, some organelles are believed to have evolved through a symbiotic relationship between eukaryotic and prokaryotic cells. Describe THREE observations that support the endosymbiotic theory.

In order to earn the full points for this question, we must first describe what the endosymbiosis theory is. This can be a succinct explanation such as:

The theory states that prokaryotic cells were engulfed by other ancestral eukaryotic cells. After the prokaryotic cells had been engulfed, they remained in the cell to produce the eukaryotic cells that we are familiar with today.

After you have explained what the theory is, the question asks for three different observations that support the theory. We have gone over a lot of evidence to support the theory; here is an example of what a full credit answer would look like.

Mitochondria and chloroplasts each contain their own genomes, allowing them to replicate on their own. The genomes of mitochondria and chloroplasts lack histone proteins that are found in the nuclear genome. Histone proteins are not found in bacterial genomes, providing evidence that mitochondria and chloroplasts may have functioned as independent bacteria prior to being engulfed by a larger cell. In addition to the lack of histones, mitochondria and chloroplasts have circular, single stranded DNA. Single stranded, circular DNA is found exclusively in prokaryotes. This evidence supports the endosymbiosis theory because these characteristics would allow the mitochondria and chloroplasts to survive on their own.

Though the answer is short and succinct, it touches on three major differences and defines the theory of endosymbiosis.


In this AP Biology Crash Course, we reviewed what the endosymbiosis theory is, the history of how the theory was formed, and the evidence that supports the theory. After reviewing the material, we answered a free response question from a previous AP Bio exam. Keep in mind while studying not only what the theory is, but of the evidence that scientists use to support it when taking your AP Biology test.

Did you enjoy this article Endosymbiosis: AP Biology Crash Course? If you did, be sure to check out the other articles on this website! If you want to continue studying the cell, checkout our other article, Cell Organelles: AP Biology Crash Course! Let us know how else you are studying for your AP Bio exam!

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Understanding the evolution of eukaryotic cellular complexity is one of the grand challenges of modern biology. It has now been firmly established that mitochondria and plastids, the classical membrane-bound organelles of eukaryotic cells, evolved from bacteria by endosymbiosis. In the case of mitochondria, evidence points very clearly to an endosymbiont of α-proteobacterial ancestry. The precise nature of the host cell that partnered with this endosymbiont is, however, very much an open question. And while the host for the cyanobacterial progenitor of the plastid was undoubtedly a fully-fledged eukaryote, how — and how often — plastids moved from one eukaryote to another during algal diversification is vigorously debated. In this article I frame modern views on endosymbiotic theory in a historical context, highlighting the transformative role DNA sequencing played in solving early problems in eukaryotic cell evolution, and posing key unanswered questions emerging from the age of comparative genomics.

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